1. Field of the Invention
The present invention relates to an optical element for uniformly irradiating X-rays in a broad area, a method for producing the optical element, an optical system using the optical element, and an optical apparatus provided with the illumination optical system.
2. Related Background Art
As a semiconductor integrated circuit element becomes finer and finer these days, there has been developed a reduction projection lithography technology using X-rays of shorter wavelength than conventionally used ultraviolet rays in place thereof in order to improve the resolving power of optical system which is limited by the diffraction limit.
An X-ray exposure apparatus used in this technology is composed mainly of an X-ray source, an illumination optical system, a mask, an imaging optical system (reduction projection optical system) and a wafer stage. A photon radiation or laser plasm X-ray source is used as the X-ray source. The illumination optical system is constituted mainly by an oblique incidence mirror for reflecting X-rays incident obliquely into a reflecting surface, a multi-layer film mirror having a reflecting surface formed of multi-layered films, and a filter reflecting or transmitting only X-rays of a predetermined wavelength, so that the system may illuminate a mask with X-rays of the desired wavelength.
The mask may be a transmitting mask or a reflecting mask. The transmitting mask is constructed such that an X-ray-absorbing material is formed in a desired pattern on a thin membrane of a material which well transmits X-rays. The reflecting mask is constructed such that low-reflectivity portions are formed in a desired pattern on multi-layered films which reflect the X-rays for example. The thus formed pattern on the mask is focused on a wafer coated with a photoresist through the reduction projection optical system comprising a plurality of multi-layer film mirrors. Since the X-rays are attenuated by atmospheric absorption, all optical paths thereof are maintained at a predetermined degree of vacuum.
Such an X-ray exposure apparatus must be so arranged that the illumination optical system irradiates (or illuminates) a broad area on the mask. For example, in case of an X-ray reduction projection exposure apparatus provided with a reduction projection optical system of 5:1 reduction ratio, the reduction projection optical system must illuminate a region of square with a side of 100 mm on the mask in order to effect exposure on a region of square with a side of 20 mm on the wafer. To obtain a desired resolving power of the diffraction limit in this case, it is desired to use the entire numerical aperture of reduction projection optical system. That is, it is desired for the rays transmitted or reflected by the mask to have an angular range which covers the angular range of entrance numerical aperture of imaging optical system (reduction projection optical system). To realize it, not only each point in the pattern surface on the mask is simply irradiated by the exposure light (X-rays), but the each point in the pattern surface on the mask must be irradiated with light having an angle of divergence corresponding to the entrance numerical aperture of exposure light of reduction projection optical system as well.
Incidentally, in case the synchrotron radiation or laser plasma X-ray source is used as the X-ray source in the X-ray exposure apparatus, the size of portion as the source radiating the X-rays (the size of X-ray source) is very small. The size of X-ray source is determined by the diameter of electron beam in case of the photon radiation, while it is determined by the spot size of laser beam irradiated onto a target in case of the laser plasma X-ray source. The obtainable size of X-ray source is about 0.1 to 1 mm in diameter in either case, which is extremely small as compared with the region to be illuminated on the mask.
Therefore, the above requirement cannot be satisfied by the conventional illumination optical system constructed of a curved-surface mirror such as the oblique incidence mirror or the multi-layer film mirror, or of a combination of such mirrors, as long as such a small X-ray source as described above is used.
For example, as shown in FIG. 1, if a mask 4 is located near an image P of a light source 2 focused by an illumination optical system 3 (this arrangement is called as critical illumination), the illumination light B is irradiated with a sufficient angle of divergence on and in the vicinity of the optical axis A of exposure light B. However, the region irradiated by the exposure light B has only the size of the image of light source 2 in a very narrow region near the optical axis A. Although the image of light source 2 can be magnified to some extent by increasing the magnification of illumination optical system 3, the angle of divergence of exposure light is inevitably reduced in that case.
On the other hand, as shown in FIG. 2, in case an image P of light source 2 is arranged to be focused by the illumination optical system 3 on the entrance pupil 50 of imaging optical system (this arrangement is called as Kohler illumination), the exposure light B can be irradiated in a considerably wide region on the mask 4. However, the divergence angle of light irradiating each point in the pattern surface on mask 4 becomes extremely small, which makes it difficult to obtain a resolving power of the diffraction limit of imaging optical system. Also, in case the mask 4 is located between the positions shown in FIG. 1 and in FIG. 2, that is, at a position between the image P of light source 2 and the entrance pupil of imaging optical system, there cannot exist together the sufficient region on the mask 4 irradiated by the exposure light B and the satisfactory angle of divergence of light irradiating each point in the pattern on mask 4.
If the size of light source 2 is nearly equal to that of mask 4 as shown in FIG. 3, rays (exposure light B) are incident in various directions even at a point distant from the optical axis A on the mask 4 and therefore a resolving power of the diffraction limit can be obtained in the region covering the entire pattern surface on the mask 4. FIG. 3 shows the case of Kohler illumination, but the illumination region is also enlarged even in the case of critical illumination, because the image of large light source is formed on the mask. It has been, however, difficult to obtain an X-ray source of such large size in actual.